The present invention relates to a Zn coated steel material, a Zn coated steel sheet and a painted steel sheet, more particularly to a Zn coated steel material, a Zn coated steel sheet and a painted steel sheet that are excellent in corrosion resistance and can be applied to various purposes, such as for home electrical appliances and building materials. The present invention further relates to a Zn coated steel sheet for construction materials and home electrical appliances that is excellent in corrosion resistance of machined portions and is planet-friendly since it does not contain chromium which is believed to put a heavy load on the environment.
Zinc-system coated steel sheet is among those most often used as a Zn coated steel material excellent in corrosion resistance. Zinc-system coated steel sheet is used in various manufacturing industries, including the automotive, home electrical appliance and building material sectors. In the building material sector particularly, Zn coated steel sheet is used without further processing for prepreg components and the like and after coating for roofing, wall materials and the like.
The need for improvement of the corrosion resistance of galvanized steel sheet used in these building material sectors is further intensifying and conventional Zn coated steel sheet is incapable of fully meeting the needs of consumers.
Galvano-aluminum steel sheet, which usually called xe2x80x9cGalvalumexe2x80x9d(copyright), (55%Alxe2x80x941.6%Sixe2x80x94Zn-alloy coated steel sheet) is therefore used as high-corrosion-resistance coated steel sheet for building materials. As peripheral patents of U.S. Pat. No. 3,026,606, that relates to this xe2x80x9cGalvalume(copyright)xe2x80x9d steel sheet, Japanese Patent Publication No. Hei 3-21627 proposes a Zn coated steel sheet having 3-20% Mg, 3-15% of Si, Al/Zn=1-1.5, and as intermetallic compound Mg2Si, MgZn2, SiO2, Mg32 (Al, Zn)49, and discloses that the corrosion resistance is good. However, owing to the fact that, similarly to xe2x80x9cGalvalumexe2x80x9d(copyright) steel sheet, the Al content of the bulk coating layer is high relative to Zn, the sacrificial corrosion prevention capability is low and the corrosivity of portions where the underlying metal is exposed, such as the end faces of coated materials, remains a problem.
On the other hand, in comparison with the method of applying a paint after first forming the steel sheet into a complex shape, painted steel sheet (precoated steel sheet) is advantageous in such points as that the painting process can be streamlined, the quality is uniform and painting material consumption is reduced, and, therefore, much has been used up to now and the amount used is expected to increase in the future. Painted steel sheet is generally formed into the desired shape after the cold-rolled steel sheet or zinc-coated steel sheet has been coated, and is then submitted to the final use. It is used in, for example, home electrical appliances (refrigerators, washing machines, microwave ovens etc.), automatic vending machines, office equipment, automobiles, the outdoor units of airconditioners, and the like.
In these various applications, the painted steel sheet is required to have an attractive appearance while also possessing machinability and corrosion resistance. In the case of products for home electrical appliances and building materials used outdoors, occurrence of corrosion at machined portions and scratched portions tends to be particularly objectionable as degrading product value, because the painted steel sheet is used after machining.
Various ways for improving the corrosion resistance of painted steel sheet have therefore been proposed. Japanese Unexamined Patent Publication No. Sho 61-152444, for instance, teaches improving fabricated-portion corrosion resistance by forming a chromate layer and a zinc-rich painting material on a Znxe2x80x94Ni coated steel sheet.
However, the foregoing and other coated steel materials, coated steel sheets and painted steel sheets disclosed up to now cannot be said to achieve sufficient corrosion resistance.
Japanese Unexamined Patent Publication No. Hei 8-168723 teaches a technology for obtaining a painted steel sheet, excellent in machinability, anticontamination property and hardness, by defining a film structure, and Japanese Unexamined Patent Publication No. Hei 3-100180 discloses a painted steel sheet improved in end face corrosion resistance by using a specific chromate treatment solution.
Such film structures are formed by subjecting a coated steel sheet of excellent corrosion resistance to a base metal treatment, called chromate treatment, that provides excellent corrosion resistance and adherence, providing an undercoating containing a chromium-system anti-rust pigment that is excellent in corrosion resistance thereon, and providing a colored overcoating on the undercoating.
The hexavalent chromium contained in the chromate-treated portion and the chromium-system anti-rust pigment is water soluble and acts to suppress corrosion of the zinc-coated steel sheet by dissolving out. If the coating should crack under harsh machining, for example, the chromium suppresses corrosion at this portion. Owing to such outstanding features, chromate treatments and chromium-system anti-rust pigments have been widely used on painted steel sheet.
However, hexavalent chromium, which may dissolve out of the chromate-treated portion and the chromium-system anti-rust pigment, is a substance that puts a heavy load on the environment. Calls for chromium-free base metal treatments and chromium-free anti-rust pigments have recently intensified.
Highly corrosion-resistant coated steel materials (steel sheet, steel wire etc.) are very likely to be used in large amounts with a view to extending service life in building material applications as well as civil engineering applications such as guardrails, sound-insulting walls, basket mats and the like. Particularly in applications such as guardrail posts and the like, whose fabrication involves roll forming, grinding with a cutting tool etc., the ordinary hot-dip galvanized steel sheet is easily scratched by the rolls and the chip from the cutting tool. On the other hand, the coating layer of Zn coated wire for basket mats is likely to develop scratches or cracks during coiling or net fabrication. As these often become a cause for degradation of corrosion resistance and the like, product improvement has been desired.
PCT/J97/04594 teaches a hot-dip Znxe2x80x94Alxe2x80x94Mg alloy galvanized steel sheet, and a method of producing the same, that is a hot-dip Znxe2x80x94Alxe2x80x94Mg alloy galvanized steel sheet good in corrosion resistance and surface appearance obtained by forming, on a surface of a steel sheet, a hot-dip Znxe2x80x94Alxe2x80x94Mg alloy galvanized layer composed of 4.0-10 wt % of Al, 1.0-4.0 wt % of Mg, Ti and B as required, and the balance of Zn and unavoidable impurities, the coating layer having a metallic structure including a primary crystal Al phase interspersed in a matrix of Al/Zn/MgZn2 ternary eutectic structure. Although this invention aims at the ternary eutectic point in the ternary state diagram and provides a steel sheet excellent in corrosion resistance, it still has room for improvement regarding the corrosion resistance of the end faces and fabricated portions.
Earlier, in Japanese Unexamined Patent Publication No. Hei 4-147955, the present inventors proposed a method of producing a Znxe2x80x94Mgxe2x80x94Al alloy galvanized steel sheet whose resistance to red rust after fabrication is markedly superior to an ordinary hot-dip galvanized steel sheet. In the present invention, the inventors have developed a Zn coated steel material, a Zn coated steel sheet and a painted steel sheet that have improved corrosion resistance of end faces and fabricated portions, and a method of producing the same. Specifically, in a Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si quaternary system, the present invention achieves high sacrificial corrosion prevention performance and enhances end-face corrosion resistance by defining a Zn-based coating layer containing 2-19% of Al, 1-10% of Mg, and 0.01-2% of Si. Sacrificial corrosion prevention performance and stabilization of corrosion products are achieved by structurally controlling the coating layer bulk portion and dispersing Mg compounds, thereby markedly improving heretofore unattainable end-face and fabricated-portion corrosion resistance.
The inventors further achieved the present invention based on the discovery that still better corrosion resistance, after coating, can be obtained by forming a Znxe2x80x94Mgxe2x80x94Alxe2x80x94Si-alloy coating on the surface of a steel material and thereafter further carrying out chromate treatment and coating. They further achieved the present invention based on the discovery that excellent corrosion resistance can be obtained, in the course of forming the Znxe2x80x94Mgxe2x80x94Alxe2x80x94Si-alloy coating on the steel material surface, by forming a metallic structure including a xe2x80x9cprimary crystal Mg2Si phasexe2x80x9d interspersed in the solidified structure of the coating layer.
Further, regarding the fabricated-portion corrosion resistance of different painted steel sheets after coating, the inventors conducted various studies under various chromium-free base metal treatment conditions and various chromium-free primer conditions. As a result, the inventors discovered that a chromium-free coated steel sheet that puts little load on the environment and has excellent coating adherence and fabricated-portion corrosion resistance can be produced by subjecting a steel sheet surface to Znxe2x80x94Mgxe2x80x94Alxe2x80x94Si-alloy coating, effecting tannin or tannin acid-system treatment instead of chromate treatment as a base metal treatment, or effecting silane coupling-system treatment instead of chromate treatment as a base metal treatment, and imparting an organic film thereon. The present invention was accomplished based on this discovery.
The inventors prepared various plating samples under differing coating bath compositions, cooling and other conditions and made a detailed investigation of the relationship between the coating layer structure and sliding property during fabrication, i.e., coating layer scratch resistance in coated steel sheet sliding tests and plated wire coiling tests, and between coating layer structure and fabricated-portion corrosion resistance. As a result, the inventors accomplished the present invention by specifying the composition and the structure the coating layer should have.
One object of the present invention is to overcome the foregoing problems by providing a Zn coated steel material, a zn coated steel sheet and a painted steel sheet that are excellent in corrosion resistance.
Another object of the present invention is to provide a Zn coated steel sheet that is excellent in fabricated-portion corrosion resistance and, being chromium free, puts little load on the environment.
Another object of the present invention is to provide a Zn coated steel material excellent in machinability, namely, a Zn coated steel material excellent in scratch resistance when subjected to sliding or coiling, adherence and fabricated-portion corrosion resistance.
The gist of the present invention is as follows:
(1) A Zn coated steel material excellent in corrosion resistance characterized in having on a surface of a steel material a Zn-alloy coating layer containing 2-19 wt % of Al, 1-10 wt % of Mg, 0.01-2 wt % of Si and the balance of Zn and unavoidable impurities.
(2) A Zn coated steel material excellent in corrosion resistance according to (1), characterized in that Mg and Al in the Zn-alloy coating layer satisfy the following formula: Mg(%)+Al(%)xe2x89xa620%.
(3) A Zn coated steel material excellent in corrosion resistance according to (1) or (2), characterized in that one or more of 0.01-1 wt % of In, 0.01-1 wt % of Bi and 1-10 wt % of Sn are further contained as Zn-alloy coating components.
(4) A Zn coated steel material excellent in corrosion resistance according to (1) or (2), characterized in that one or more of 0.01-0.5% of Ca, 0.01-0.2% of Be, 0.01-0.2% of Ti, 0.1-1.0% of Cu, 0.01-1.0% of Ni, 0.01-0.3% of Co, 0.01-0.2% of Cr, 0.01-0.5% of Mn, 0.01-3.0% of Fe and 0.01-0.5% of Sr are further contained as Zn-alloy coating components, that total amount of elements other than these elements is held to not greater than 0.5 wt % and that among them Pb is limited to not greater than 0.1 wt % and Sb to not greater than 0.1 wt %.
(5) A Zn coated steel material excellent in corrosion resistance according to (1) or (2), characterized in that the coating layer has a metallic structure of primary crystal Mg2Si phase, MgZn2 phase and Zn phase interspersed in a matrix of an Al/Zn/MgZn2 ternary eutectic structure.
(6) A Zn coated steel material excellent in corrosion resistance according to (1) or (2), characterized in that the coating layer has a metallic structure of primary crystal Mg2Si phase, MgZn2 phase and Al phase interspersed in a matrix of an Al/Zn/MgZn2 ternary eutectic structure.
(7) A Zn coated steel material excellent in corrosion resistance according to (1) or (2), characterized in that the coating layer has a metallic structure of primary crystal Mg2Si phase, MgZn2 phase and, Zn phase and Al phase interspersed in a matrix of an Al/Zn/MgZn2 ternary eutectic structure.
(8) A Zn coated steel material excellent in corrosion resistance according to (1) or (2), characterized in that the coating layer has a metallic structure of primary crystal Mg2Si phase, Zn phase and Al phase interspersed in a matrix of an Al/Zn/MgZn2 ternary eutectic structure.
(9) A Zn coated steel material excellent in corrosion resistance according to any of (1) to (8), characterized in that a Ni coating layer is formed as an underlying layer for the Zn-alloy coating layer.
(10) A Zn coated steel material excellent in corrosion resistance and machinability according to any of (1) to (4), characterized in that a Mg-system intermetallic compound phase of a major diameter of not less than 1 xcexcm is dispersed in the Zn-alloy coating layer at a content of 0.1-50 vol%.
(11) A Zn coated steel material excellent in corrosion resistance and machinability according to (10), characterized in that the intermetallic compound phase containing Mg is one or more of Mgxe2x80x94Si-system, Mgxe2x80x94Zn-system, Mgxe2x80x94Sn-system, Mgxe2x80x94Fe-system, Mgxe2x80x94Ni-system, Mgxe2x80x94Al-system and Mgxe2x80x94Ti-system.
(12) A Zn coated steel material excellent in corrosion resistance and machinability according to (10) or (11), characterized in that a Ni coating layer is formed at 0.2-2 g/m2 as a base metal treatment for the Zn-alloy coating layer.
(13) In the method of producing a Zn-alloy coated steel material having on a surface of a steel material a Zn-alloy coating containing 3-10 wt % of Mg, 4-19 wt % of Al, 0.01-2 wt % of Si and the balance of Zn and unavoidable impurities, a method of producing a Zn coated steel material excellent in corrosion resistance characterized in that plating bath temperature is set at not less than 450xc2x0 C. and not greater than 650xc2x0 C. and a cooling rate after coating is controlled to not less than 0.5xc2x0 C./second.
(14) A Zn coated steel sheet excellent in corrosion resistance according to any of (1)-(12), characterized in that it has, as an upper layer on the Zn-alloy coating layer, a resin chromate film of 10-300 mg/m2 as metallic chromium formed by applying and drying a resin chromate bath that utilizes a water-soluble chromium compound of a chromium reducibility {CR3+/(CR3++Cr6+}xc3x97100(wt %)} of not greater than 70 (wt %), is adjusted to a copresence of phosphoric acid and the water-soluble chromium compound such that a H3PO4/CrO3 ratio (as chromic acid) is not less than 1 and a H3PO4/Cr6+ ratio (as chromic acid) is not greater than 5, and is blended with an organic resin to make an organic resin/CrO3 ratio (as chromic acid) not less than 1.
(15) A painted steel sheet excellent in corrosion resistance according to any of (1)-(12), characterized in that it has as an intermediate layer on the Zn-alloy coating layer, a chromate film layer and, further, as an upper layer an organic film layer of 1-100 xcexcm thickness.
(16) A painted steel sheet excellent in corrosion resistance according to (15), characterized in that the organic film is a thermosetting resin coating film.
(17) A painted steel sheet that is excellent in fabricated-portion corrosion resistance and puts little load on the environment according to any of (1)-(12), characterized in that it has, on the Zn-alloy coating layer, an intermediate layer containing 100 parts by weight of resin as solids content and 0.2-50 parts by weight of tannin or tannic acid, and has an organic film layer as an upper layer.
(18) A painted steel sheet that is excellent in fabricated-portion corrosion resistance and puts little load on the environment according to any of (1)-(12), characterized in that it has on the Zn-alloy coating layer an intermediate layer containing 100 parts by weight of resin as solid content and 0.1-3000 parts by weight of a silane coupling agent, and has an organic film layer as an upper layer.
(19) A painted steel sheet that is excellent in fabricated-portion corrosion resistance and puts little load on the environment according to (17) or (18), characterized in that the organic film layer has a thickness of 1-100 xcexcm.
(20) A painted steel sheet that is excellent in fabricated-portion corrosion resistance and puts little load on the environment according to (17), characterized in that the intermediate layer further contains 10-500 parts by weight of fine-grain silica as solid content.
(21) A painted steel sheet that is excellent in fabricated-portion corrosion resistance and puts little load on the environment according to (18), characterized in that the intermediate layer further contains at least one of 1-2000 parts by weight of fine-grain silica and 0.1-1000 parts by weight of an etching fluoride as solid content.
(22) A painted steel sheet that is excellent in fabricated-portion corrosion resistance and puts little load on the environment according to any of (17)-(21), wherein the organic film layer is composed of an undercoating containing an anti-rust pigment and a colored overcoating.